1. Field of the Invention
The present invention relates to semiconductor diode lasers and, more particularly, to diode lasers that are capable of emitting a plurality of optical modes at different wavelengths with low relative intensity noise and optical transmission systems utilizing these lasers.
2. Description of Related Art
The use of optical signals having different wavelengths results in greatly increased information carrying capacity by employing dense wavelength-division-multiplexing (DWDM). This technique requires an optical source which is capable of providing a sufficiently large number of signals having different wavelengths (channels). It is currently a common practice to build an optical source for a DWDM system using a plurality of single-frequency distributed feedback lasers (DFBs) 901 generating the signals at different precise stable wavelengths (FIG. 1). Each signal is then modulated by the modulators 902 and then transmitted to the couplers 903. The couplers 903 are used to couple the output signals from the modulators to an AWG multiplexer 904 that combines the signal from the individual DFB lasers. Then, the combined signal is outputted from the AWG multiplexer 904 to a coupler 905 and then comes to an output optical fiber 906. The signals can be transmitted between different elements of the optical transmission system (DFB lasers, modulators, couples, etc) by optical fibers 906.
The complexity of fabrication of a DFB laser 901 results in its high cost, which is further multiplied by the number of lasers 901 to be integrated. Furthermore, an operation wavelength should be independently determined for each single-frequency laser 901. This makes it difficult to build an array of single-frequency lasers 901 with pre-selected wavelength separation. In addition, each laser 901 requires its own system of wavelength stabilization. Considering the possible integration of a multiwavelength laser source with silicon optoelectronic components for future computing, an array of DFB lasers 901 has the further disadvantage of a large footprint. As the number of required optical channels increases and the wavelength separation between channels decreases, all the aforementioned problems of a conventional optical source are enhanced.